An oscillator based active bridge circuit for converting capacitance change into frequency for capacitive humidity sensor

A simple signal conditioning circuit for converting capacitance change into frequency for capacitive humidity sensors is presented. It is based on a relaxation oscillator in which output frequency is linearly related to the capacitive unbalance of an active bridge. The design, analysis, and experimental results of the circuit and its application to a sol gel thin film porous γ-Al₂O₃ based humidity sensor are reported. Experimental results confirm the theoretical value predicted. The circuit covering wide capacitance measurement range has the potential for remotely monitoring measurement parameters accurately.